The present invention relates to a super twisted nematic type liquid crystal display device.
In a super twisted nematic type liquid crystal display device the twist angle of the liquid crystal layer thereof is changed from 90.degree., which is the twist angle of the liquid crystal layer of a prior art twisted nematic type liquid crystal, to another angle over 180.degree.. For this reason, in the super twisted nematic type liquid crystal display device, change of alignment of liquid crystal molecules accompanied by rise of the driving voltage is abrupt and change of transmittance accompanied by rise of the driving voltage is also abrupt. In the super twisted nematic type liquid crystal display device, since highly multiplex driving using X-Y electrodes can be effected, it is widely utilized as an image display device for a lap-top type word processor, a personal computer, etc.
The maximum value of the ratio, which two-valued driving voltages V.sub.1 and V.sub.2 applied to a pixel can take, is determined unequivocally by the number of scanning lines N of the X-Y electrodes (matrix electrodes) used at effecting a multiplex driving. This relationship is given by a following equation [1]; ##EQU2## where it is supposed that V.sub.1 &lt;V.sub.2.
FIG. 21 indicates the dependency of the electric capacity of a liquid crystal cell in the super twisted nematic type liquid crystal display device on the driving voltage. As indicated in FIG. 21, the electric capacity of the liquid crystal cell varies rapidly, starting from a certain driving voltage. The inventors of the present invention call the voltage at this time a threshold voltage Vth, which is defined as a following equation [2]; EQU Vth=(Co-Ca+.alpha.Va)/.alpha. . . . [2]
where C.sub.o represents the electric capacity of the liquid crystal cell, when an AC driving voltage V.apprxeq.0 of frequency 1 kHz is applied to the liquid crystal cell; V.sub.a the driving voltage, when the variation in the electric capacity of the liquid crystal cell is greatest, accompanied by the rise of the driving voltage (frequency 1 kHz); C.sub.a the electric capacity of the liquid crystal cell, when the variation in the electric capacity of the liquid crystal cell is greatest, accompanied by the rise of the driving voltage; and .alpha. a rate of variation in the electric capacity of the liquid crystal cell, when the variation in the electric capacity of the liquid crystal cell is greatest, accompanied by the rise of the driving voltage.
The dependency of the electric capacity of the liquid crystal cell on the driving voltage for the super twisted nematic type liquid crystal display device indicated in FIG. 21 reflects the dependency of the alignment of liquid crystal molecules on the driving voltage. FIG. 22A indicates the dependency of the statistical average .theta. of the angle formed by the molecular axis of liquid crystal molecules in the liquid crystal cell and the surface of a substrate on the driving voltage of the super twisted nematic type liquid crystal display device. .theta. is usually below 10.degree., when a driving voltage below the threshold voltage Vth is applied thereto, but increases rapidly in the neighborhood of the threshold voltage Vth. Further, on the side of the driving voltage higher than the threshold voltage Vth, variations of .theta. are slow.
FIG. 22B indicates the dependency of the response time of the super twisted nematic type liquid crystal display device on the driving voltage. The abscissa in FIG. 22B represents V.sub.1, when the response time is measured, supposing that V.sub.1 &lt;V.sub.2 and that the ratio of V.sub.1 to V.sub.2 is constant (in this example, value corresponding to N=200 in Eq. [1]). The response time is longest in the neighborhood of the threshold voltage Vth and it has a value of about 450 ms at that time. Further the response time is shortened on the side of the driving voltage higher than the threshold voltage Vth.
Here, in the present specification, the response time is defined as indicated in FIG. 1. When V.sub.1 &lt;V.sub.2, the driving voltage is varied between V.sub.1 and V.sub.2. The electric capacity of the liquid crystal cell is varied, accompanied thereby, and the amount of this variation is denoted by .DELTA.C (=C.sub.2 -C.sub.1). The rise time T.sub.R is defined as a period of time from the moment, where the driving voltage is changed-over from V.sub.1 to V.sub.2 to the point of time, where the amount of variation in the electric capacity reaches 0.9 .DELTA.C, while the fall time T.sub.F is defined as a period of time from the moment, where the driving voltage is changed-over from V.sub.2 to V.sub.1, to the point of time, where the amount of variation in the electric capacity reaches 0.9 .DELTA.C. The response time is defined as the sum of the rise time T.sub.R and the fall time T.sub.F.
FIG. 22C indicates the dependency of the transmittance of a prior art super twisted nematic type liquid crystal display device on the driving voltage. This is a super twisted nematic type liquid crystal display device called blue mode. No optical anisotropic layer is mounted on either side of the liquid crystal cell and the product .DELTA.n.sub.LC.d.sub.LC of the thickness d.sub.LC of the nematic liquid crystal layer by the birefringence .DELTA.n.sub.LC of the nematic liquid crystal is set at about 0.8 .mu.m. The dependency of the transmittance on the driving voltage thus obtained is as indicated in FIG. 22C. As it can be seen therefrom, the transmittance varies rapidly in the neighborhood of the threshold voltage Vth and variations in the transmittance are slow on the side of the voltage higher than the threshold voltage Vth. The contrast ratio is one of parameters representing the display quality of the super twisted nematic type liquid crystal display device together with the brightness. As clearly seen from FIG. 22C, for the contrast ratio a satisfactory value can be obtained, only when the two-valued driving voltages V.sub.1 and V.sub.2 are set in the neighborhood of the threshold voltage Vth. As clearly seen from FIG. 22C, if the two-valued driving voltages V.sub.1 and V.sub.2 are set on the side of the voltage higher than the threshold voltage Vth, since variations in the transmittance are slow, no satisfactory contrast ratio can be obtained. Consequently, the prior art super twisted nematic type liquid crystal display device should be driven under the driving voltage corresponding to the longest response time. That is, in the prior art super twisted nematic type liquid crystal display device the response time and the display quality cannot be compatible.
Contrarily thereto, in another prior art super twisted nematic type liquid crystal display device, an optical anisotropic layer is disposed between a liquid crystal cell and a polarizer to intend an improvement of the dependency of the transmittance on the driving voltage (JP-A-63-151924). However the product .DELTA.ni.di of the thickness di of the optical anisotropic layer by the birefringence .DELTA.n.sub.i is set so as to be almost equal to the product .DELTA.n.sub.LC.d.sub.LC of the thickness d.sub.LC of the nematic liquid crystal layer by the birefringence .DELTA.n.sub.LC of the nematic liquid crystal. Further the value of .DELTA.n.sub.LC.d.sub.LC is set at about 0.8 .mu.m. FIG. 23 indicates the dependency of the transmittance on the driving voltage thus obtained. The scale of the abscissa in FIG. 23 is the same as that used in FIGS. 22A, 22B and 22C. Compared with FIG. 22C, although variations in the transmittance in the threshold voltage Vth are more rapid, variations in the transmittance on the side of the voltage higher than the threshold voltage Vth are as slow as those indicated in FIG. 22C. Consequently it can be driven only in the neighborhood of the threshold voltage Vth. Also in this case, the response time and the display quality cannot be compatible.
Further, in still another prior art super twisted nematic type liquid crystal display device, an optical anisotropic layer (compensating liquid crystal cell) having .DELTA.ni.di different from .DELTA.n.sub.LC.d.sub.LC is used (JP-A-64-49021). The value of .DELTA.n.sub.LC.d.sub.LC is set at about 0.9 .mu.m. FIG. 24 indicates the dependency of the transmittance on the driving voltage thus obtained. The scale of the abscissa in FIG. 24 is the same as that used in FIGS. 22A, 22B and 22C. When the case indicated in FIG. 23 is compared with the case indicated in FIG. 22C, the driving voltage, for which the transmittance is smallest, is shifted towards the side of the voltage higher than the threshold voltage Vth. For this reason, both of the bright state and the dark state can display on the side of the voltage higher than the threshold voltage Vth and it is possible to drive the liquid crystal display device on the side of the voltage higher than the threshold voltage Vth, where the response time is shortened. However, the increase in the transmittance on the side of the voltage higher than the smallest point of the transmittance is slow and it is impossible to obtain a satisfactory brightness and contrast ratio. If it is impossible to obtain a satisfactory brightness and contrast ratio, it cannot work as a display device. Also in this case, the response time and the display quality cannot be compatible.
Further, in still another super twisted nematic type liquid crystal display device, the product .DELTA.n.sub.LC.d.sub.LC is set at a value over 1.1 .mu.m (JP-A-2-118516). However, no relation between .DELTA.n.sub.i.d.sub.i and .DELTA.n.sub.LC.d.sub.LC is defined. FIG. 25 indicates the dependency of the transmittance on the driving voltage obtained when .DELTA.n.sub.i.d.sub.i and .DELTA.n.sub.LC.d.sub.LC are set so as to be equal to each other. The scale of the abscissa in FIG. 25 is the same as that used in FIGS. 22A, 22B and 22C. The region of the driving voltage, where variations in the transmittance are abrupt, is extended on the higher voltage side. However there exists no minimum on the side of the voltage higher than the threshold voltage Vth. Or, even if there exists, the transmittance at that time doesn't decrease sufficiently. For this reason, it is impossible to drive the liquid crystal display device on the side of the voltage higher than the threshold voltage Vth. Also in this case, the response time and the display quality cannot be compatible.